1. Field of the Invention
This invention relates to power-driven valve actuators and, more particularly, to valve actuators which provide for optional manual operation, which provide for automatic disengagement of drive power, which provide adjustable electrical torque switches, which provide adjustable mechanical torque-overload protection and which provide adjustably positionable limit switches for operation of the valve connected thereto.
2. Prior Art
Various types of power-driven means for actuating valves are known. One such motor-driven device is shown in U.S. Pat. No. 2,203,233 which discloses a worm gear mechanism for operating a valve. To avoid straining or breaking the mechanism due to improper valve seating or an obstruction, a torque sensor detects axial movement of a worm shaft and cuts off electrical power to the drive motor when the reaction torque on the worm shaft exceeds a certain value. This particular torque sensor is operative in one direction only and cannot be used to detect both opening and closing torques in a valve. In the case of electrical power failure or a failure of the electrical drive motor, no provisions are made for manual operation of the valve.
Limit switches are often used to sense the position of a valve and to control operation of the actuator for the valve; however, such limit switches have not been readily adjustable to meet different valve operating conditions.